This invention relates generally to the field of expanding gate valves and deals more particularly with an improved guide arrangement for an expanding gate assembly.
Expanding gate valves typically include a gate member and a cooperating segment having interfacing wedge surfaces which act against one another to expand the gate mechanism by wedging action in both the fully open and fully closed positions of the valve. In order to prevent undue dragging or binding of the gate and segment against the valve seats, it is necessary to maintain the gate assembly in a collapsed condition as it moves between the open and closed positions. It is also necessary to assure that the gate assembly expands properly when it reaches the fully open or fully closed position.
In the past, curved springs have often been used to maintain the gate mechanism in its collapsed condition between the open and closed positions, as shown in U.S. Pat. No. 2,954,960 for example. When springs are used, their force must be overcome before the gate assembly can expand as intended, and the operating torque of the valve is increased accordingly. In addition, the forces exerted by the springs tend merely to move the gate and segment together. Consequently, when the gate assembly is initially moved away from the open or closed position, it is common for "back wedging" to occur whereby the gate moves beyond the intended centered position relative to the segment. The gate assembly is thus expanded prematurely due to wedging action between interfacing surfaces which are not intended to wedge against one another in the collapsed condition of the gate mechanism. Such back wedging is highly undesirable in that it greatly increases the operating torque of the valve and causes excessive wear on the valve components.
U.S. Pat. Nos. 2,583,512 and 2,855,175 disclose gate centralizers which avoid the forementioned problems associated with springs. However, the complicated mechanisms which are required in centralizers of this type add significantly to the cost and complexity of the valve. Another type of device for properly positioning an expanding gate assembly is disclosed in pending application Ser. No. 950,494, filed Oct. 11, 1978 by P. D. Alvarez.